ApoE is a polymorphic protein in humans and promotes the clearance of lipoproteins remnants. There are three common alleles (ε4, ε3, and ε2) that encode apoE in humans. The three isoforms (apoE4, apoE3, and apoE2, respectively) result from mutations at amino acid residues 112 and 158. ApoE4 contains arginine at both positions. ApoE3 contains cysteine at residue 112 and arginine at reside 158. ApoE2 contains cysteine at both positions. There also exists three rare alleles: apoE1 (G127D/R158C); apoE2* (R145C); and apoE2** (K146Q).
ApoE is a component of VLDL, IDL, HDL, chylomicrons and chylomicron remnants, and is required for the clearance of lipoprotein remnants from the circulation. Lipoprotein-bound apoE is the ligand for the LDL receptor as well as other LDL receptor family members and SR-BI. In vitro and in vivo studies have shown that the apoE2 isoform and other apoE mutants that prevent binding of apoE-containing lipoproteins to the LDL receptor are associated with high plasma cholesterol levels and cause premature atherosclerosis in humans and experimental animals. ApoE promotes cholesterol efflux and thus may contribute to cell and tissue cholesterol homeostasis and protection from atherosclerosis. ApoE is also a risk factor for Alzheimer's disease and may contribute to lipid homeostasis in the brain.
It was shown that overexpression of full-length apoE (by infection of mice with 1-2×109 pfu) did not correct the high cholesterol levels of the apoE−/− mice; in contrast, it increased VLDL triglyceride secretion and induced hypertriglyceridemia. Overexpression of apoE3 or apoE4 also aggravated the hypercholesterolemia in apoE2 knock-in mice. The high cholesterol profile of apoE−/− mice or the apoE2 knock-in mice was corrected by infection with truncated apoE forms lacking different segments of the C-terminal domain. The hypertriglyceridemia induced by full-length apoE was independent of the apoE phenotype and mouse strain and could be corrected by overexpression of lipoprotein lipase. In normal C57BL/6 mice overexpression of full-length apoE induced combined hyperlipidemia, characterized by high cholesterol and high triglyceride levels.
Previous in vitro experiments have shown that residues 260-269 of apoE are important for binding of apoE to lipids and lipoproteins. Use of a series of apoE deletion mutants extending from amino acid 1 to amino acids 185, 202, 229 or 259 mapped the region responsible for the hypertriglyceridemia between amino acids 260-299 of apoE. Deletion of residues 260-299 of apoE diminished greatly the ability of the truncated apoE to solubilize multilamellar dimyristoyl-L-α-phosphatidyl-choline (DMPC) vesicles. Further, deletion of residues 166-299, 203-299, or 230-299 completely eliminates the ability of apoE to solubilize multilamellar DMPC vesicles. Thus, the carboxy-terminal 260-299 amino acids of apoE is involved in the initial association of apoE with phospholipid, a process that may be required for the formation of apoE-containing lipoproteins. Once apoE is lipoprotein-bound, it may be taken up by the LDL receptor. The contribution of receptors other than the LDL receptor in the clearance of apoE-containing lipoprotein remnants was previously assessed by studies in apoE−/−×LDLr−/− double-deficient mice (Kypreos, et al., 2003). However, neither the full-length apoE2 or apoE4 nor the truncated apoE2-202 (deletion of residues 203-299) or apoE4-202 corrected the high cholesterol profiles of the apoE−/−×LDLr−/− double-deficient mice. Thus, in the absence of this receptor, lipoprotein receptor related protein (LRP) and heparan sulfate proteoglycans are not sufficient to clear the lipoproteins remnants, which accumulate in the plasma of the double-deficient mice (Kypreos, et al., 2003).